3.8.5 is the latest Glade supporting GTK2, all newer Glade Versions do only support GTK3, so at this development state they are not compatible with GladeVCP.

Chris has done some work on porting GladeVCP to GTK3, but the Preview part is not solved, as GTK3 does not support any 3D preview window. All possibilities I found do work only with work around or use toolkits what are no longer supported.

IMHO we should all bundle our enforcements and change from pyGTK to pyQt.
As soon as I have finished porting gmoccapy to master (joint Axis support) I will begin to build a new GUI special for retrofitted machines and will try to base that one on pyQt. So any hand will be welcome

I dabbled with PyQT a few times but to me it is quite confusing to get a handle on it. I wish I understood it better. Maybe better is C++ and Qt I guess... I can write some pretty intense programs with python and gtk. I've even written a golang dxf to g code converter that is unreal fast... I need to stop rambling and take a nap.

GTK2 is dead, there are no more development, as it has been replaced with GTK3, so we will be forced in the very near future to
transfer all gtk2 stuff to gtk3 or all the beautiful GUI based on that will not work any more on modern OS.

PyVCP is IMHO an very old relict and not any more "up to date" I have not checked, if there is still support for that one. But editing XML Files by hand is not "user friendly". Do we want users to add stuff, or just developers? IMHO we live in a click drag and drop world.

PyQt : is well supported, there are debs for a lot of Linux variants and does support GUI design by drag and drop and python is easy to learn.

So this is a basics discussion and the will to change and to begin with the change.
In my business we say, the one who is not willing to accept changes is not able to survive!